Since everyone so far are going the more realistic ARM/RISC-V route, I'm going to go with something more out there.
First, commission WDC to make some improvements to their 65C816 softcore design to pull in functionality of the never produced 65C832. The performance issues that killed the project 35ish years ago don't really apply to a "alt-history" machine, and since we care less about backwards compatibility, I would like a true 32-bit PC and databus, and a flat-address space(I'm fine with jettisoning banking completely). I think being having a relatively simple CISC was a large part of what made the 6502 so easy to pick up, so I think keeping that part of the lineage is important.
When combined with softcore implementations based on the specs and any remaining documentation for the Advanced Amiga Architecture chipset, I think it would result in a very interesting machine, albeit not backwards compatible with either the C64 or Amiga families.